Rare and relatively expensive to extract, xenon is one of a family of seven noble gases; like helium, krypton, neon, and argon, it does not play well with other elements.

That aloofness should act as a sort of safety blanket, protecting xenon from being used up in chemical reactions. So, according to Elissaios Stavrou of the Lawrence Livermore National Laboratory, its deficiency in Earth’s atmosphere is “difficult to explain.”

Stavrou and an international team of researchers—hailing from the Carnegie Institute for Science, Stony Brook University, University of Saskatchewan, University of Chicago, and Lawrence Livermore Lab—looked for the missing xenon deep inside the Earth.

Specifically hidden in compounds with nickel and iron, which forms most of the planet’s core.

While xenon doesn’t usually form compounds under normal conditions, when lost in the extreme temperatures and pressures of planetary interiors, it lets down its guard a bit.

“When xenon is squashed by extreme pressures, its chemical properties are altered, allowing it to form compounds with other elements,” Sergey Lobanov of Stony Brook University, explained.

So the researchers put their theory to the test, mimicking conditions found within the Earth’s core to see how xenon interacts with nickel and iron.

“Our study provides the first experimental evidence of previously theorized compounds of iron and xenon existing under the conditions found in the Earth’s core,” Carnegie’s Alexander Goncharov said in a statement.

But there’s a catch: “It is unlikely that such compounds could have been made early in Earth’s history,” Goncharov said, “while the core was still forming and the pressures of the planet’s interior were not as great as they are now.”

Researchers are investigating a two-stage formation process, which may have trapped xenon in Earth’s early mantle before merging with iron to form XeFe3.